Westerlies

A Perth listener rang in to ask how come the wind always blew from west to east, or from Perth to Sydney. After all he said, the Earth rotates in that direction, and if anything, you'd expect the wind to go the other way as the Earth rotated underneath it.

The answer is Conservation of Momentum - as surface winds travel towards the Pole, they also get closer to the North-South axis of the Earth, AND they have to go west, to conserve momentum.

Let me explain. Air rises at the equator until it hits warm air - 15 kilometres above the ground. It heads to about 30° south of the Equator, and then falls down to the ground, and then heads, at ground level, to 60° South.

As it goes from 30° South to 60° South, it gets closer to the Pole AND the North-South axis of the spinning Earth. It's just like the skater on ice spinning, with their arms held away from their body. As they bring their arms in, they have to conserve their angular momentum, so they speed up their rate of spin. As the wind moves closer to the Pole, and the axis of the Earth, it speeds up its rate of west-to-east spin. And that's how come you get the westerlies.

In the Northern hemisphere, the westerlies don't really get a chance to strut their stuff, because they're mostly interrupted by land. But in the Southern hemisphere, between 40° and 50° south, there's hardly any land. There the westerlies build up so much force that they're called the Roaring 40s. In the old days, a clipper ship could make 3,000 kilometres in a week.